Field of the Invention
The present invention relates to compositions useful as corrosion inhibitors in CO.sub.2 absorption systems and more particularly to compositions for inhibiting corrosion of metallic surfaces exposed to alkanolamines used in such absorption systems. This invention finds particular usefulness in upgrading systems for removing CO.sub.2 from recycled reducing gases used in processes for the direct reduction of iron ore to sponge iron, which may also contain H.sub.2 S, COS and the like.
Typical gaseous reduction systems for the production of sponge iron are exemplified in U.S. Pat. Nos. 3,765,872; 4,336,063; and 4,428,772. The latter two specifically incorporate CO.sub.2 removal units in the disclosed process.
Such CO.sub.2 absorption systems are widely known. A general description thereof can be found in the text "Gas and Liquid Sweetening" (Second Edition, April 1977) by Dr. R. N. Maddox, published by John M. Campbell (Campbell Petroleum Series) specifically, Chapter 3 on "Amine Processes".
The need for CO.sub.2 absorber units with greater efficiency, longer life, greater capacity, lower operating costs, and the ability to process gas streams containing higher acidic concentrations has caused the industry to recognize the need to increase the concentration of the absorbent, i.e. aqueous alkanolamine. See for example, the first paragraph of U.S. Pat. No. 4,071,470. By increasing the concentration of alkanolamine, greater CO.sub.2 absorption per volume of solution can theoretically be achieved. However, there is a corresponding increase in the corrosion of the metallic components of the absorber units, particularly in the reboiler, the absorber column and the associated piping of the regeneration portions of the absorber where the steel is exposed to a hot, protonated alkanolamine solution.
Several alternatives have been suggested by the prior art to minimize the corrosive effects of alkanolamine absorbents in CO.sub.2 absorption systems. One suggestion is to use stainless steel or corrosion resistant alloys as materials of construction for the contact surfaces. This, of course, results in prohibitively high capital costs. Another alternative is to provide for a side-stream reclamation still to remove corrosive degradation products thereby exercising greater control over the process conditions. See, for example, Pearce et al., U.S. Pat. No. 4,477,419, Column 1, lines 35-47. Another proposed solution is to add corrosion inhibiting compounds to the absorbing solution to inhibit corrosion and/or the formation of corrosive elements. See, for example, the paragraph in Pearce et al., column 1, at lines 66, et seq. for the use of copper as an inhibitor. See also "Corrosion Control in CO.sub.2 Removal Systems", Chemical Engineering Progress, Vol. 69, No. 2, February 1973. This article discusses various approaches, some of which are exemplified in the patents discussed below. The specifically disclosed inhibitor has been found not to be very effective in the presence of sulfur.
In contrast, U.S. Pat. No. 4,143,119, Asperger et al., discusses in the first column, the relative ineffectiveness of copper alone as an inhibitor. It then discloses as a corrosion inhibiting composition the combination of "copper ions in the presence of sulfur atoms" (or copper and a sulfur salt plus an oxidizing agent to assure the presence of free sulfur) used in an alkanolamine solution.
Asperger, et al. discloses a series of experiments made with different copper sources and different oxidizing agents combined with H.sub.2 S. In Table II the use of a zinc permanganate salt is shown with CuCO.sub.3 and, separately, with Cu (NO.sub.3).sub.2. This is taught in conjunction as a solution of 80% of monoethanolamine in water saturated with H.sub.2 S at room temperature.
Asperger, et al. did not disclose or teach any special inhibiting properties as being attributable to any ingredient beyond copper and effectively free sulfur. The purpose of the experiments in Table II was to test different oxidizers, for example: permanganates, persulfates, peroxides, etc.
U.S. Pat. No. 4,096,085, Holoman Jr.. et al., discloses a corrosion inhibited composition consisting of (1) an amine compound, (2) copper, and (3) sulfur.
U.S. Pat. No. 4,071,470, Davidson et al., discloses a method for inhibiting corrosion of metals in contact with an absorbent inhibiting composition by reacting monoethanolamine, copper, sulfur and an oxidizing agent.
U.S. Pat. No. 1,989,004, Fife, discloses a process for removing sulfur compounds wherein some metals such as copper, iron, nickel and zinc are added to the alkanolamine solution in order to enhance the absorption of sulfur. Fife makes no reference to corrosion problems. U.S. Pat. No. 3,959,170, Mago, discloses a compound of antimony and vanadium which, when added to the alkanolamine solution, results in corrosion inhibition.
U.S. Pat. No. 2,559,580, Alexander, discloses that iodine must be present in the alkanolamine solution in order to prevent the degradation of the amine in which copper is present.
Some of these are effective in treating sour gases having higher concentrations of H.sub.2 S relative to CO.sub.2 but are taught as best being effective for gases contaminated only with CO.sub.2 (e.g. see Asperger). Others show some improvement over the use of Cu alone as an additive, but require still greater enhancement.
It is an object of the present invention to provide a CO.sub.2 removal process which can be used in a plant constructed with less expensive carbon steel and can be operated at higher concentrations of alkanolamine resulting in less energy loss and higher CO.sub.2 absorption efficiency.
It is a further object to provide a commercial corrosion inhibitor composition which effectively passivates metallic iron surfaces during normal CO.sub.2 plus H.sub.2 S scrubbing operations.
It is another object to provide a process for achieving the foregoing objectives with respect to a reducing process gas such as used for the production of sponge iron.